One of the problems in solution or slurry polymerization of monomers is the capital costs required in the production thereof. Monomer polymerization using a vapor phase process cn be considerably more economical if certain problems inherent in vapor state polymerization can be solved. These include problems of carrying out the polymerization in a thermally controlled fashion so as to avoid hot spots, maintaining a proper product particle size distribution and, in the case where catalysts are utilized which have extremely high yields but are extremely sensitive to poisoning, decreasing to a minimum the amount of make-up material seen by the catalyst per amount of product formed. Now we have discovered a reactor off-gas treatment process which largely solves the above referred to problems and realizes important economic benefits through savings in energy consumption, raw materials and capital equipment.
In U.S. Pat. No. 2,936,303 (Goins) a vapor state polymerization of ethylene, propylene or mixtures thereof is carried out in a countercurrent, fluidized bed in the presence of inert diluent gas. In the process described therein catalyst in solid form is passed downwardly in the reactor and, for example, ethylene mixed with diluent gas is passed countercurrently up through a series of vertical fluidized bed reaction zones. In this process the reaction in the various reaction zones can be controlled independently by taking off-gas from the last reaction zone, cooling it, and recycling portions of such off-gas to each of the reaction zones. However, because of the use of diluent gas and a fluidized bed large quantities of gas must be passed through the reactor per unit of polymer produced.
In U.S. Pat. No. 3,023,203 (Dye) a suspended, gas phase polymerization of olefins using a reactor having three concentric superimposed vertical sections, comminuted catalyst addition suspended in the entering gases and temperature regulation by gas cooling is shown. The process includes polymer removal without pressure letdown.
In U.S. Pat. No. 3,254,070 (Roelen) a method for the gas phase polymerization of ethylene using conventional cooling is described wherein a mixture of reaction products and solid or liquid catalysts is constantly being mechanically subdivided. This patent additionally teaches that the reacting material may be agitatingly moved through a number of stages each with different polymerization conditions. An additional mode described therein teaches that the first few stages may be carried out using liquid phase polymerization finishing up with polymerization in the vapor phase. Different polymerization temperatures may be used in the separate reactors.
In U.S. Pat. No. 3,256,263 (Wisseroth et al.) a method of removing heat and producing intensive movement of the polymeric product produced during the gas phase polymerization of olefins in a stirred, vertical, fluidized reactor is described using mechanical agitation and introduction of the polymerization monomer immediately after expansive cooling in the form of moving gas or liquid jets.
In U.S. Pat. No. 3,300,457 (Schmid et al.) polymerization of mono-olefins, particularly ethylene and propylene, is accomplished in a fluidized bed using a stirred, vertical reactor. Catalysts and polymer in the reactor are moved in the direction of flow of the mono-olefin by the stirring and heat is removed by cooling the walls of the vessel, by conduction using the gas stream, or by the introduction of liquefied mono-olefin.
In U.S. Pat. No. 3,469,948 (Anderson et al.) a horizontal, paddle type polymerization reactor using pulsed gases including the polymerization olefin is described. The teaching of this patent is directed to the use of longitudinally oriented paddles which are suitably shaped to give forward movement to the solid polymer resulting from the vapor phase polymerization.
In U.S. Pat. No. 3,652,527 (Trieschmann et al.) a process for the gas phase polymerization of propylene in a stirred-bed, vertical reactor using evaporative cooling of the reaction is described. This patent teaches that it is essential that one component of the catalyst combination must be directly placed on the bed of solid polymer produced in the polymerization and the second catalyst component must be introduced in the gas phase above the bed.
Finally, in British Patent No. 1,354,020 (Badische) a vapor phase olefin polymerization process is described in which the heat generated in the polymerization process is removed by the introduction of the monomer and heat transfer agents in gaseous or liquid state in the polymerization zone. However, the use of multiple polymerization temperatures in the vertical, stirred, fluid-bed reactor is not taught and it is difficult to see how much individual control could be accomplished in the apparatus described.